Scanning probe microscopes (SPMs) such as atomic force microscopes (AFMs), in particular, image a surface by applying a thin probe tip at the end of a flexible cantilever to a surface and moving the probe tip over the surface. As the probe tip is moved over the surface, the changing topography of the surface results in height variations of the probe tip which are typically detected by the use of an optical detector sensing the position of a reflected laser spot on the cantilever. Deflections in the cantilever due to height variations of the probe tip move the position of the reflected laser spot on the optical detector. In a constant force mode application, a servo-system is used to keep movement of the reflected laser spot on the optical detector to a minimum by moving the actuator that controls the cantilever to keep the cantilever deflection approximately constant. Because the deflection of the cantilever is due to the interaction between the probe tip and the surface, keeping the deflection approximately constant is equivalent to keeping the force approximately constant. Typically, the cantilever is controlled by a piezoelectric actuator in the prior art.
Because the optical detector is responsive only to the deflection of the cantilever and not the absolute position of the tip, the typical control loop for the servo-system is an output error only loop. Therefore, the output signal from the controller of the servo-system is used as an estimate of the surface topography. This typically limits the bandwidth and accuracy of the position estimate to that of the control loop itself.